The structure includes a low voltage chamber of transformer and an auto compensation unit of wattless power. Characters are that the structure also includes a cage type cabinet. With parts and apparatus being installed, the integrated cage type cabinet through a supporting rack is installed on the front wall of the low voltage chamber. The method includes steps: assembling parts and apparatus in auto compensation unit inside the cage type cabinet; carrying out first and second time of debugging wiring; installing the cage type cabinet on the front wall of the low voltage chamber. Advantages are: easy of installation and debugging, high production efficiency; high carrying capacity of front wall of the low voltage chamber; and using space of low voltage chamber reasonably.